The present invention relates to separating fibers from lignocellulosic materials, such as the separating fibers from wood chip feed material. The invention particularly relates to mechanical refining, including chemi-mechanical pulping (CMP) and thermomechanical pulping (TMP).
In some conventional mechanical refining processes, the steps of defibration and fibrillation are performed together in a single mechanism. The benefits of separating the steps of wood fiber defibration and fiber fibrillation are discussed in, for example, U.S. Pat. No. 7,300,541 ('541 patent), which is based on published international patent application PCT/US03/22057. When fibers are defibrated prior to fibrillation, the primary refining step may be optimized for fibrillation. The optimization for fibrillation may be to minimize energy dissipation by increasing refining intensity. A method to separate the defibration and fibrillation steps is described in the '541 patent a using a pressurized chip press followed by gentle refining to separate fibers in a pretreatment stage (referred to as a “defibration step”) and thereafter by high intensity pressurized primary refining stage (the “fibrillation step”).